When preserving biological tissue such as biological cells for a relatively long time, the biological tissue is filled into a freezing bag and is cryopreserved by immersing the freezing bag filled with the biological tissue in liquid nitrogen. The freezing bag includes a bag main body which is formed in a bag shape by fusing a plastic film or films. The bag main body is equipped with a filling port and a discharge port. The biological tissue is suspended in a cryopreservation solution or the like, and the suspension is filled into (i.e., introduced into) the bag main body through the filling port to cryopreserve the biological tissue. At the time of using the biological tissue, the freezing bag preserved in liquid nitrogen is taken out and thawed, and the biological tissue is taken out via the discharge port to be used. Japanese Patent Application No. 2003-205016 discloses an example of this process to extract, thaw and use the biological tissue.
Cells may be damaged if biological tissue is frozen by abruptly immersing the biological tissue in liquid nitrogen. Japanese Patent Application No. 2003-267471 thus explains that it is preferable to freeze the biological tissue by cooling it down to −80° C. at a rate of approximately 1° C./minute to 5° C./minute by use of a programmable freezer or the like. On the other hand, at the time of thawing the frozen biological tissue, a method of warming the frozen biological tissue by immersing it in hot water at 37° C. to achieve rapid thawing or a similar method is generally adopted for reducing damage to the cells, as described in Japanese Patent Application No. 2002-253206.
The freezing bag may be broken by mistake by the operator at the time of cooling, freezing, preservation, warming, thawing or the like because the freezing bag is produced using a fragile blank material such as plastic film. In addition, the freezing bags are immersed in liquid nitrogen and frozen in a mutually adhered state and, therefore, it is difficult to take the freezing bag out of liquid nitrogen. In order to solve such a problem, bags and containers for protecting the freezing bags have been developed.
For instance, Japanese Patent Application No. 2003-267471 and Japanese Patent Application No. 2000-140069 each disclose a covering bag, which is obtained by forming a film with excellent impact resistance into a bag shape to protect a freezing bag, and a metallic container for accommodating the covering bag covering the freezing bag. The covering bag and the metallic container are designed for consistently protecting and storing the freezing bag throughout the above-described process of freezing and thawing the cells in the freezing bag. At the time of freezing, however, part of the freezing bag may be rapidly cooled, such that cells in the freezing bag may be frozen non-uniformly. At the time of thawing, on the other hand, the freezing bag may fail to be appropriately (i.e., sufficiently) warmed up, and insufficient thawing of the biological tissue may occur, resulting in a lowered recovery rate.